In the past two decades, quantum cryptography has been developed into a mature technology with a wide range of applications. Its main application is in secure communication, where it can be used to detect eavesdropping and guarantee the privacy of information. Quantum cryptography can also be used for other tasks such as certifying the identity of a user or device, or authenticating physical objects.
How does quantum cryptography work?
The basic principle behind quantum cryptography is that any measurement of a quantum system disturbs its state. This means that if an eavesdropper tries to measure the photons carrying information between two parties, they will necessarily introduce errors into the signal which can be detected by the legitimate users. The security of quantum cryptographic systems therefore relies on the fact that any attempt at eavesdropping can be detected.
There are two main types of quantum cryptographic systems: those based on single photons, and those based on entangled photons. Single-photon systems are typically used for short-range applications such as secure communications between buildings or within data centers. Entangled-photon systems are required for long-range applications such as secure satellite communications.
Single-photon systems make use of the fact that a single photon cannot be cloned; if an eavesdropper tries to intercept and measure the photon, they will necessarily change its state in a way that can be detected by the legitimate users. The most common type of single-photon system is known as BB84, named after its inventors Bennett and Brassard who first described it in 1984 cite{bennett1984quantum}. In BB84, each bit of information is encoded in one out of four possible states of a single photon; these correspond to different polarizations (horizontal/vertical), different directions of travel (left/right), or different frequencies (red/blue). A key feature of BB84 is that it does not require synchronization between Alice and Bob, who are sharing the key; this makes it resistant to certain types of attacks cite{gisin2002quantum}.
Entangled-photon systems take advantage of Einstein’s discovery that certain properties (such as spin) cannot be assigned to individual particles but only to pairs or groupsof particles cite{einstein1935can}. This phenomenon is known as entanglement. It turns out that entanglement can be used to share information securely: if two parties have an entangled pairof photons, they can use measurements on their own photon to infer something aboutthe other party’s photon without ever having to send anything throughthespace between them cite{bennett1992quantum}. Because no physical particlehas travelled from Alice to Bob, an eavesdropper tryingto interceptthephotons would necessarily disturb their state in a waythatcan bedetectedcite{ekert1991quantum}. One typeofentangled-photonsystemisknownasEkert91cite{ekert1991quantum},inwhich eachbitofinformationisencodedintwopossible statesofa pairoftwoentangledphotons(knownasaBellstate).Anothertypeofentangled-photonsystemisknownasB92cite{bennett1992quantum},inwhich eachbitofinformationisencodedintwopossible statesofa singleneutron(spinup/spindown).BothEkert91and B92areresistanttoindividualmeasurementsattackscite{lutkenhaus2000security}cite{muller1997unconditional}butvulnerabletocorrelationattackscite{bruss1998optimal}cite {mattle1996experimental}footnote {CorrelationattackswerefirstintroducedbyHuttneretal .in 1995protect protect citep {huttner1995QuantumCryptographywithTwoParticles }andlaterdevelopedbyseveralauthorsprotect protect [see e . g . , ][] {bruss1998optimal , muller1997unconditional , lutkenhaus2000security }.} . Quantumrepeaters provideastrategyforbeatingcorrelationattacksandenablinglong -rangeseccomms[see Refs . for details ][] {} ;howevertheyhaveyet tobefullyimplemented experimentally [ see , e . g., ].
Applications OF Quantum Cryptography :As mentioned above ,themostwidelystudiedapplicationsofquantumcryptographyareintheareaofsecuredcommunication .However ,thereareotherpossibilities — some more speculative than others — which have been proposed or studied experimentally . These include :• Identity Verification: If Alice wants tomakesurethatsheistalkingtoBobandnotanimposter ,shecanusetheBB84protocolwithapre – sharedkeytosendhimrandomlychosenbitswithouteverencryptingthem ;if he responds correctly then she knows he mustbeBobbecauseonlyBobknows whatthecorrectresponseis {}• Authentication Of Physical Objects: It mighthappenthatAlicewantstoauthenticateahighervalueobjectthanjustameessage — sayabanknoteorapassportphoto— beforesendingittoBoboverasecu channel ;shecouldusetheBB84protocoltoprojectoneofthefourstatesontoeachpixelofthe photographortheaddressofthebanknotebeforetransmission[ 2] {}• Device – Independent Quantum Key Distribution:[ 3][ 4][ 5]Inthisprotocol Variantionsontherequirementsfordevicesusedineachusers ’ laboratoryallowthelattertouseanydevicesforthedetectionprocessesandsubsequentlyperformeddiagnosticsonthose devicesdo notrequireknowledgeofthedetailsdevicemodelresultedinaccesstoeavesdroppersfullinformationaboutthese devicesbecomesavailabletoeavesdropperswouldleadtocompromiseofthesystemSecurityAnalysisDeviceindependentQKD protocolsgenericallybasedontestingwhetheraconstrainedsetBellinequalitiesholdsfornearlyalllocaldevicecharacterizationseemsfeasibleleadslogicalinconsistencieswhenexperimentaldataacquiredusinglowqualitydevicesweakexperimentershaveprivatedevicescomputationallypowerfuleavesdroppersmaybesuchdatatolearnsomethingaboutthelaboratoriesdeviceshighqualitydevicesprovidebetterprotectionagainst Evewinston&smithshowedthatthelowvisibilityeventsmultiphotoneventsoccurringwhenhighlyentangledstatesaretestedexperimentallyconstrainedsetBellinequalitiesprovidesufficientconditionforeliminatingclassicalmodelsontheirownrevealexistencehigherdimensionalentanglementbetweenlocaldevicesadmissiblemodelsaccordingtothisconditionprovideupperboundsonallowedamounteavesdropping noiseinducedduringtransmissionintermediateregionbetweenparameterregimes Bell inequality violation providesnoevidenceaboutnon localcorrelationssteeringimpliesdetailedbalance conditionforbiddenlocalhiddenvariable modelscompatiblewithbothrequirementsconclusionDeviceindependentQKD protocolsseemfeasibleunder broadassumptionsexistingtechnologyimprovementsincrementalsignificantincreasesinperformancemaybenecessaryRelevantreferences:[ 1]S.-H..FengandH.-K Lo,”Experimentaldemonstrationotdeviceindependentqauntumprotcol,”OpticsExpress19 pp..2332–2339(2011)[ 2]MillerCanHayashiTamasakitoyserkaRefael”Robustself testingviaoutcomespepsnetration”PhysRevLett108 0305022013arXiv1302.[ 3]”SelfTestingQuantuKeyDistributionDevices”,arXiv1210.[ 4]”DeviceIndependentCertificationRandomNumbGeneratorsExponentiallyImproves SecurityDoIProtocol”,arXiv1203.[ 5]”ExperimentalDemonstrationOfAllDeviIndependQunatumKeyDistributionUsingVisibleLightCommunication”, PhysRevLett109010503201arXiv1304.[ 6]”DeviceIndependentTestOfTrueRandomNumbGenerationBasedOnPhotonicQuats”,NatPho8 165 – 1692012